This invention relates to an X-ray image intensifier and to a process for its production.
X-ray image intensifiers essentially consist of an input screen, or scintillator, with which a photocathode is associated, an electron-optical system and a so-called output screen on which the image appears in visible light. The X-photons impinge on the scintillor which produces luminous photons which are in turn absorbed by the photo-cathode emitting electrons. The electrons are accelerated by the electron-optical system and impinge on the output screen where they give rise to an emission of visible light.
The input screen, or scintillator, is generally formed by a layer of cesium iodide (CsI) doped with sodium in proportions which are optimised so as to obtain maximum luminescence when the screen is subjected to excitation by X-rays.
This CsI layer has a thickness of from 150 to 200 .mu.m selected in such a way as to obtain a compromise between, on the one hand, the X-absorption of the material, which should be as high as possible so as to ensure a maximum conversion of X-photons into luminous photons which necessitates considerable thicknesses and on the other hand the resolution of the luminescent layer, this resolution being higher, the narrower the thickness of the layer.
For a thickness of the CsI layer of 150 .mu.m, approximately 40% of the X-photons of 60 KeV are absorbed.
This doped CsI layer is generally deposited onto an aluminium substrate in the form of a spherical cap. The processes currently used for depositing this CsI layer include:
The co-evaporation in vacuo of CsI and sodium iodide (NaI) from a single heated crucible onto the hot substrate kept at a temperature of from 200.degree. to 250.degree. C.;
co-evaporation by exactly the same process, but onto a cold substrate, followed by heat treatment at a temperature above 250.degree. C.
In either case, however, the material obtained after evaporation does not have the property of luminescence under X (or UV) excitation if there has been no heat treatment during or after the evaporation step.
The structure of a layer thus obtained is far from favourable to the production of a high-resolution screen because, irrespective of whether evaporation is carried out on a hot substrate or on a cold substrate, followed by heat treatment, the temperature acts on the deposited material by intercrystalline diffusions. This results in the formation of isotropic monocrystals of fairly large dimensions (from 100 to 200 .mu.m in the lateral directions) possibly separated by cracks which insulate them electrically and optically from one another to a greater or lesser extent.
FIG. 1 shows part of a CsI layer which has been deposited by one of the processes described above and which has been subjected to the heat treatment required for its effectiveness. It can be seen that this layer is not uniform, but instead is formed by the juxtaposition of monocrystals 1.
FIG. 2 illustrates the behaviour of such a layer under the effect of X-radiation. It is assumed that a CsI layer consisting of monocrystals 1 deposited onto a substrate 2 receives a beam 3 of X-rays coming from a narrow slot 4 formed in a screen 5 exposed to the radiation. An X-photon 6 emanating from the beam 3 is absorbed by the CsI and produces several luminous photons which emerge from the monocrystal 1 after having undergone a certain number of different reflections and refractions. Some emerge directly from the monocrystal while others undergo one or more reflections before emerging therefrom so that the incident X-ray beam, irrespective of its size, will give rise to the emission of a luminous radiation in all directions as if it were a substantially omni-directional light source equal in its dimensions to a monocrystal 1, i.e. between 100 and 200 .mu.m. The image of the slot will thus be enlarged.
It can be seen that a structure such as this makes it impossible to obtain a high-resolution screen. For a layer thickness of 150 .mu.m, the resolution limit obtained does not exceed 6 to 7 pairs of lines per millimeter.
In order to reduce the dimensions of the monocrystals responsible for the lateral diffusion of the luminous radiation created under X-excitation, attempts have been made artificially to creat cracks in the luminescent layer by utilising the difference between the coefficients of expansion of the CsI layer and the substrate on which it is deposited. However, the regions spatially separated from one another still have excessive dimensions (approximately 60 .mu.m) and it is difficult to obtain by this process a resolution of better than 8 pairs of lines per millimeter. In addition, the cracks thus obtained create electrically insulated islets to the detriment of the lateral electrical conductivity of the photocathode.
The present invention relates to an X-ray image intensifier which does not have any of these disadvantages.
The invention also relates to a process for producing this X-ray image intensifier.
The layer of cesium iodide is formed by needles of small diameter (3 to 8 .mu.m) obtained by vapour condensation on a cold substrate. A foreign material is introduced into the crystal lattices of the cesium iodide to prevent the needles from coalescing into monocrystals of large dimensions during the subsequent heat treatment.
According to the invention, the X-ray image intensifier consisting of a scintillator with which a photocathode is associated, an electron-optical system and an output screen on which the visible image appears, is characterised in that the scintillator is formed by a substrate onto which a layer of cesium iodide doped with sodium is deposited, said layer having a structure of fine needles that are substantially parallel to and optically independent.
According to the invention, this result is obtained by the following steps:
alternately depositing onto the cold substrate by condensation vapours of the actual scintillator material, cesium iodide doped with sodium, and vapour of a material different from cesium iodide;
heat treating the deposit thus applied by increasing the temperature of the assembly to between 250.degree. and 500.degree. C.
Other features will become apparent from the following descriptions of particular embodiments and explanations in conjunction with the accompanying drawings, wherein: